Liquid crystal or mesomorphic compositions are of increasing interest in a variety of electro-optic display devices. Liquid crystal compositions are of particular interest for electrically controllable, flat panel displays such as watch faces, digital clocks, calculator displays, numeric displays for instruments and the like. An electro-optic device, e.g. a liquid crystal cell, comprises a layer of a liquid crystal composition between two closely-spaced parallel electrodes, at least one of which is transparent. In a preferred embodiment, the electrodes comprise glass plates having a transparent, conductive film thereon, as of tin oxide or indium oxide, suitably patterned for the desired display. When the electrode plates are connected to a source of voltage, an electric field is generated in the liquid crystal composition.
Field effect liquid crystal devices contain nematic compounds or mixtures of liquid crystal compounds having positive dielectric anisotropy. The electrode plates have been treated so that the liquid crystal molecules align themselves in a particular direction, usually parallel, to the plane of the plates. When an electric field is applied, the positive dielectric anisotropy of the molecules causes them to realign themselves in a direction parallel to the applied field and perpendicular to the plates. The change in alignment is made visible using a polarizer and an analyzer on either side of the cell. Field effect liquid crystal cells have the advantages of lower threshold voltages and wider viewing angle than other electro-optic devices such as dynamic scattering cells, and they have good contrast and long lifetimes.
Several positive anisotropy liquid crystal compounds having carboxylate linkages are known. For example, U.S. Pat. No. 3,925,237 to Ross et al. discloses positive liquid crystal compounds of the formula ##STR4## wherein X can be alkyl, alkoxy, acyloxy or alkylcarbonato and the alkyl groups have 1 to 10 carbon atoms.
U.S. Pat. No. 3,925,238 to Gavrilovic discloses positive liquid crystal compounds of the formula ##STR5## wherein X can be alkyl, alkoxy, acyloxy or alkycarbonato wherein the alkyl groups have 1 to 10 carbon atoms, and Y is cyano or nitro.
U.S. Pat. No. 3,951,846 to Gavrilovic discloses positive liquid crystal compounds of the formula ##STR6## wherein X can be hydrogen, alkyl, alkoxy, acyloxy or alkylcarbonato wherein the alkyl groups have 1 to 10 carbon atoms.
Copending application of Gavrilovic, Ser. No. 696,904 filed June 17, 1976, discloses compounds of the formula ##STR7## wherein X is alkoxy, acyloxy or alkycarbonato and the alkyl groups have 1 to 10 carbon atoms.
Each mesomorphic compound has a particular temperature range in which it is an ordered liquid, ranging from the solid to nematic liquid crystal melting point, up to the temperature range useful in electro-optic cells. Although, as is known, wide variations in useful temperature ranges can be effected by employing mixtures of known liquid crystal compounds that are compatible with each other, no single liquid crystal compound or mixture of compounds now known can satisfy all of the useful temperature ranges that are desired. The particular mesomorphic temperature range for each compound, or even whether a compound will be mesomorphic or not, is on the whole unpredictable. Thus, new liquid crystal compounds which have different useful temperature ranges are being sought to satisfy various temperature requirements for which the liquid crystal cells will be employed.